And sorry it's a premultiply problem :-). In NUKE its works directly without changing a single option, but I prefer to use Fusion. I added a screenshot for comparison, also the source Fusion .comp is included.

I tried al the .settings related to this at vfxpedia, but the outlines remain. even with boolean operators..nothing works.

Can anyone open the comp file and have a look? The goal is to have the same result as in NUKE, so without the white outline.

The pot has edge pixels which are brighter than their corresponding alpha channels. This creates the bright edge when it's being merged. The reason it doesn't show up in Nuke is because the tif is in sRGB color space. Nuke removes the sRGB gamma when it reads the image, Fusion just reads the image as it is...

You can fix it in Fusion, but to me this looks like the tif is already slightly flawed. Alpha (un)premultiplication hasn't been done correctly when it was converted to sRGB.

Is that tif file coming from Fusion/Nuke or has it been rendered like that in a 3D app?

What do you mean by "there is no more alpha"? That the alpha channel has disappeared?!

It did in my previous comp, when I applied a gamut, my alpha was completely white over the entire frame, but I was still able to put layers behind it. AFAIK it was just a viewport glitch because now it works like it should.

Also, when color correcting a single layer after the gamut I have to enable Pre Divide/Post Multiply in the CC options to avoid a small fringe. When enabled the result is perfect.

In Fusion 7 we'll be able to adjust an sRGB gamut in the loader itself, that's very good news.